The present invention relates to a liquid metal ion source of the type utilized in a secondary ion mass spectrometer, a focused ion beam processing device, etc., for generating a converged ion beam having a high intensity and a micro-spot diameter.
For example, Japanese utility model laid-open application No. 63-101452 discloses a conventional liquid metal ion source of the type which ionizes a liquid metal by means of a high electric field. FIG. 1 is a simplified sectional view of such a source.
In this source, a liquid metal 1 is contained and held in a reservoir 2. An emitter 3 of the needle type has a sharp tip end which protrudes from the reservoir 2. Typically, the needle type emitter 3 is composed of tungsten (W). Further, at least a part of the needle type emitter 3 is immersed in the liquid metal 1. A heater 4 is disposed around the reservoir 2 so as to heat and melt the metal within the reservoir 2 to form the liquid metal 1. The liquid metal 1 is supplied to the tip end of the needle type emitter 3 due to the wettability of emitter 3. The heater 4 is covered by a reflector 5 to reflect thermal radiation to improve heat efficiency.
The liquid metal 1 within the reservoir 2 flows along the surface of the needle type emitter 3 due to its wettability to reach the sharp tip end of emitter 3.
An extracting electrode 6 is provided in opposed relation to the sharp tip end of the needle type emitter 3 such that an ion extraction voltage of the order of 2-10 kV is applied between the emitter 3 and the extracting electrode 6. By this extraction voltage, liquid metal ions are emitted from the tip end of the needle type emitter 3.
In an ion source with such a needle type emitter, since the liquid metal 1 flows along the emitter surface to feed the tip end of the emitter 3, the wettability of the liquid metal 1 with respect to emitter 3 is a significant factor. Namely, if the liquid metal 1 has poor wettability with respect to the needle type emitter 3, liquid metal ions are not generated efficiently. Further, in case that the liquid metal has a high vapor pressure at the temperature of the liquid metal ion emission, the liquid metal 1 is vaporized considerably at the tip end portion of the emitter 3 and will then be deposited on an insulating portion of the ion source device, thereby practically disadvantageously causing insulation failure and interior contamination.
However, the origin of emission of ions is confined within the sharp tip end area of the emitter 3 so that there can be obtained an ion beam which is converged to a tiny spot diameter.
There is another type of emitter structure as shown in FIG. 4 where the emitter is of a capillary type. The capillary type emitter 3a is formed as a minute tube at the bottom of reservoir 2 such that the liquid metal 1 flows through the tube to thereby emit ions from the tip end of the capillary type emitter 3a. In this structure, even if the liquid metal 1 has a relatively poor wettability to tube 3a, the liquid metal 1 can be fed to the tip end of the capillary type emitter 3a. Therefore, this structure can be applied to liquid metals having a relatively poor wettability.
Further, since the capillary tube is almost sealed except at the tip end of the emitter 3a, there can be utilized a metal material having a relatively high vapor pressure. However, as the level of metal in the ion emission source descends progressively, the beam diameter varies. However, depending on the changes of the portion of the ion emission source where the ionization occurs, the beam drifts.
In a secondary ion mass spectrometer (SIMS), an ion bun utilizes cesium (Cs) as an ion species. The use of the cesium ions as the primary ion species can significantly improve the negative secondary ion yield of negative elements such as carbon (C), oxygen (O), Fluorine (F), chlorine (Cl), sulfur (S) and selenium (Se). Therefore, cesium is suitable for the ion species of the ion gun in such secondary ion mass spectrometer.
The cesium metal ion source can improve the secondary ion yield when analyzing negative elements by a SIMS as mentioned above, hence there can be obtained a spectrometer having a very high sensitivity. However, liquid cesium does not have a good wettability for a material of the needle type emitter such as tungsten, hence liquid cesium cannot be fed smoothly to the tip end of the needle type emitter from the reservoir in the ion source device. Further, cesium metal has a high vapor pressure which may cause spark or short failures due to deposition of cesium on an insulating portion of the ion source device as a result of evaporation of the cesium. Namely, there has not been provided a practical liquid cesium metal ion source having a needle type emitter which could operate stably for a long time in a high vacuum condition for performance of highly sensitive analyses.
In view of this, conventionally, the capillary type emitter has been utilized practically in liquid cesium metal ion sources. As described before, the capillary type emitter has the drawbacks that the beam spot diameter varies gradually and the ion beam can not be converged to a tiny spot diameter because the ions are emitted from a relatively wide area.